Shielded cables are typically classified as flexible, semirigid or rigid, with cables having greater rigidity typically having more predictable electrical properties. A flexible shielded cable usually has a shield formed of braided copper. While such a shield may perform satisfactorily at low frequencies, the openings in the braid permit high frequency energy transfer thus limiting the use of such cables.
A common type of semirigid coaxial cable includes a copper tubing into which the core assembly (made up of the central conductor and its dielectric jacket) is inserted. This type of coaxial cable is relatively expensive because it is not manufactured in a continuous process. A length of the core assembly is inserted into a length of tubing, and the tubing is shrunk by swaging resulting in a tight fit. While the formed copper tubing does provide a smooth, continuous inner shield surface for effective shielding over a wide frequency range, it has severe mechanical shortcomings. This type of coaxial cable is relatively heavy, not very flexible, and special tools are required for bending without kinking or breaking the shield. The use of the copper tubing, which has minimum elasticity, also limits the maximum operating temperature to the cable.
A recently developed coaxial cable includes a layer of conductive or semi-conductive matter surrounding the dielectric. A shield, which may be a braid, is embedded in the layer which is softened by heating. For further information regarding the structure and operation of this cable, reference may be made to U.S. Pat. No. 4,486,252.
Another type of coaxial cable, described in U.S. Pat. No. 4,694,122, includes a layer of foil surrounding the dielectric, braided shield over the foil, and molten material bonding the braid and foil. A problem with this structure is that the braiding operation is relatively slow.